Display screen or portion thereof with a graphical user interface



The FIGURE is a front view of a display screen or portion thereof with a graphical user interface, showing our new design.

The dotted and evenly-dashed broken lines, specifically, 24 circles of a 5-by-5 array of circles, a rectangular bottom banner including an outermost broken line that illustrates a boundary of a display screen or a portion thereof, and all icons and text in the banner, illustrate portions of the graphical user interface that do not constitute part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a graphical user interface, as shown and described. 